The present invention relates to an electrolytic reduction cell for the production of a metal, such as aluminium. The invention particularly relates to a cathode construction used in such cells.
Aluminium metal is generally produced by the Hall-Heroult process in which electrical current is passed through an electrolytic bath comprising alumina dissolved in molten cryolite to cause the electrodeposition of molten aluminium. Electrolytic reduction cells comprise an outer steel shell that is lined with a layer of insulating material, such as refractory bricks. Carbonaceous blocks are placed on top of the insulating layer and these carbonaceous blocks form the cathode of the cell. The cathode must last for the expected operating life of the cell, which is typically 1000 to 2000 days. A number of consumable anodes are located a short distance above the cathode. In use, the electrolytic bath is located between the cathode and the anodes and the passage of electrical current through the cell causes molten aluminium to form at the cathode. In conventional cells, the molten aluminium collects as a pool on top of the cathode and in operation the pool of molten aluminium acts as the top of the cathode. Aluminium is periodically drained from the cell, typically on a daily basis.
Electrolytic reduction cells are arranged in potlines in which a large number of cells are connected in series. Electrical current enters a cell through the anodes, passes through the electrolytic bath and pool of molten metal and into the cathode. The current in the cathode is collected and passes to an external current carrier and then along to the next cell.
In conventional aluminium reduction cell technology, embedded collector bars are used to collect electrical current from the carbonaceous cathode and conduct it to the external ring bus. The embedding of collector bars, which is performed with the use of cast iron or carbonaceous glue, imposes a number of limitations which adversely affect service life, cost and performance of aluminium reduction cells.
Accommodation of collector bars within the cathode carbon requires a machined groove to be formed in the block and thus increases the cost of cathode blocks and at the same time, the presence of a groove reduces the potential cell life (available erodable lining), in some cases by about 40%. Furthermore, the cathode current density distribution along the length of the cathode blocks is uneven with the outer-most portions of the cathode blocks drawing current at up to three to four times higher density compared to the inner portions of the block.
In embedded collector bar technology, the bar is either cast or glued into a recess on the underside of the cathode block. Under normal operating conditions the electron transfer from the collector bar to the carbon occurs through active spots (a-spots) which are concentrated along the sides of the collector bar and nearest to the block end. The top portion of the collector bar normally does not participate in electron transfer as its own weight and a lack of high-temperature strength causes it to sag. The concentration of a-spots along the sides of the collector bar slots increases the average current path length in the cathode carbon and thus increases cathode voltage loss.
Most of the current transfer from collector bars to carbon occurs near the block end and this leads to uneven current distribution on the surface of the cathode. It is highest nearest to the outer edge of the anode shadow or ledge toe. The uneven cathode current density has a dual effect on cell operation: on the one hand it increases the rate of dissolution of carbon by increasing the chemical activity of sodium (this drives the aluminium carbide forming reaction) in the affected region, and on the other, it increases the rate of transport of dissolved aluminium carbide by inducing circulation of metal and catholyte. This increased circulation can result either from the increased metal pad heave due to interaction in the metal pad of horizontal currents with the vertical magnetic fields or from the Maragonni effect (i.e. circulation induced by uneven interfacial tension between catholyte and aluminium due to uneven cathode current density distribution at the interface). The rate of erosion of carbon is directly related to the rate of circulation of metal and catholyte.
As neither the horizontal currents in the metal pad, nor the interacting magnetic fields are even, balanced, or static, their coupling can lead to hydrodynamic instability of the metal-bath interface. The circulation of the metal, the deformation of its surface and the instability of the metal-bath interface, are the three most significant limitations of the current technology cells which affect their potlife (cathode and sidewall erosion) and operating efficiency.
In conventional current feeding technology it is difficult to build a reduction cell which can have a completely uniform cathode current density distribution throughout the cell. The best which can be achieved is to reduce the variation of current density distribution by constructing relatively narrow cells, using relatively deep, high resistivity, anthracitic cathode blocks and using large steel collector bars. The problem of metal heave and metal pad stability (product of field current interaction) was then addressed through the modification of bus bars to control the vertical magnetic field. Modem magnetically compensated cells are a good example of this type of engineering within the limitations of the system.
This problem of cathode current density distribution and the presence of horizontal currents in the metal pad has restricted the cell design to construction of relatively narrow, but long reduction cells. Such furnace designs are at a disadvantage as they have a high external surface to production volume ratio, hence have a high heat loss. In conventional cell construction methods, these limitations resulting from embedded collector bar technology have been accepted as inherent to the nature of the aluminium reduction cell cathode and its negative impact was minimised by focussing on improving the magnetic field aspect of the current/field interaction. Modem reduction cells are designed with magnetic compensation in order to improve the hydrodynamic stability of the cells. However, this requires relatively expensive external bus bars.
In a paper published in Aluminium, 70, Jahrgang, 1994, pp 105-109, Lakomsky, one of the present inventors, described sources of electrical resistance in an electrolytic reduction cell. In particular, in cells there is invariably electrical contacts at interfaces between steel based conductors and carbonaceous materials. Such contacts occur, for example, at the collector bar/cathode carbon interface. Collector bars are typically mounted into a slot formed in the bottom of the cathode carbon block and molten cast iron is poured around the collector bar. Although the cast iron wets the steel collector bar to ensure very good contact therebetween, the molten cast iron does not wet the carbonaceous material of the cathode. Accordingly, the cast iron and cathode carbon do not form a continuous electrical joint. The two solid surfaces do not make contact over the entire surface area but rather at discreet points, called a-spots. Passage of electrical current through the a-spots depends on overcoming the contact resistance in each of the contact materials near the a-spots. The greater the number of a-spots, the lower the contact resistance.
This paper further describes a method of improving the contact of carbon material with metal such that contact resistance is reduced. The method involves welding the contacting parts together so that permanent joints are established that block the access of air or other oxidising agent to the interface and hence prevent oxidation at the interface. The welded joint more importantly increases the actual contact area between the metal and the carbonaceous material to thereby reduce the contact resistance.
Such welded joints were embodied in the Lakomsky paper by "electrical contact plugs" welded into a carbonaceous material. The diametral section of such an electrical contact plug is shown in FIG. 5 of Lakomsky. The plug diameter and height were chosen to provide a tight contact of the plug to the carbon material over the entire contact boundary, whilst ensuring that no cracking resulted from metal shrinkage during solidification in the plug, no cracking in the carbon layers close to the plug due to thermal stresses and no failures in the fusion line due to the difference in the thermal expansion coefficients of the dissimilar material. It was found that plugs of 30 mm diameter and depth were the most useful.
The electrical contact plugs were mounted in the slot formed in the cathode carbonaceous material that accepts the collector bar. In particular, the plugs were welded into the block body on the horizontal slot surface. The cathode carbon with electrical contact plugs mounted thereto were joined to steel collector bars by a standard method using molten cast iron. Apart from using electrical contact plugs, the assembled cathode blocks did not differ in any way from standard cathode blocks.
In mounting the steel collector bar in the slot in the cathode block, the molten cast iron wets both the surface of the collector bar and the open surface of each electrical contact plug. This forms "bridges" of lower electrical resistance between the carbon block and the collector bar. Operation of cells in a plant environment incorporating a cathode constructed as described above resulted in a cathode voltage drop of 40-50 mV, when compared to plug-free cells. In the plant at which the trials were conducted, this resulted in a saving of 130-170 kWh per tonne of metal produced.
The present invention provides an improved cathode construction for an electrolytic smelting cell.